Abstract
The question "Can ions exert supra-additive effects on water dynamics?" has had several opposing answers from both simulation and experiment. We address this ongoing controversy by investigating water reorientation in aqueous solutions of two salts with large (magnesium sulfate) and small (cesium chloride) effects on water dynamics using molecular dynamics simulations and classical, polarizable models. The salt models are reparameterized to reproduce properties of both dilute and concentrated solutions. We demonstrate that water rotation in concentrated MgSO4 solutions is unexpectedly slow, in agreement with experiment, and that the slowdown is supra-additive: the observed slowdown is larger than that predicted by assuming that the resultant of the extra forces induced by the ions on the rotating water molecules tilts the free energy landscape associated with water rotation. Supra-additive slow down is very intense but short-range, and is strongly ion-specific: in contrast to the long-range picture initially proposed based on experiment, we find that intense supra-additivity is limited to water molecules directly bridging two ions in solvent-shared ion pair configuration; in contrast to a non-ion-specific origin to supra-additive effects proposed from simulations, we find that the magnitude of supra-additive slowdown strongly depends on the identity of the cations and anions. Supra-additive slowdown of water dynamics requires long-lived solvent-shared ion pairs; long-lived ion pairs should be typical for salts of multivalent ions. We discuss the origin of the apparent disagreement between the various studies on this topic and show that the short-range cooperative slowdown scenario proposed here resolves the existing controversy.
Highlights
IntroductionThe effect of salts on any given solution property strongly depends on the identity of both ions but, for a given counterion, it is possible to create a series of anions or cations ranked by their impact on any given property
The model predicts water dynamics well at the lowest CsCl concentration tested here, 0.5 m, where 85% of water molecules are considered in the bulk, and at the highest CsCl concentration, 2.5 m, where only 25% of the water molecules are in the bulk. These results suggest that water dynamics in CsCl solutions of concentrations higher than 2.5 m but low enough that each ion still retains an intact first hydration layer may still be understandable as the sum of contributions from isolated ions
Our prior work suggests that another effect may be at play in MgSO4 solutions: we have shown that isolated pairs of magnesium and sulfate ions with 5, 6 or 7 Å inter ionic distance may supra-additively slow-down water rotational dynamics.[31]
Summary
The effect of salts on any given solution property strongly depends on the identity of both ions but, for a given counterion, it is possible to create a series of anions or cations ranked by their impact on any given property. These ionic series are commonly known as Hofmeister series.
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